Phosphors are used for a vacuum fluorescent display (VFD), a field emission display (FED), a plasma display panel (PDP), a cathode ray tube (CRT), a white light-emitting diode (LED), and the like. In all these applications, it is necessary to provide energy for exciting the phosphors in order to cause emission from the phosphors. The phosphors are excited by an excitation source having a high energy, such as a vacuum ultraviolet ray, an ultraviolet ray, an electron beam, or a blue light to emit a visible light. However, as a result of exposure of the phosphors to the above excitation source, there arises a problem of decrease of luminance of the phosphors and hence a phosphor exhibiting no decrease of luminance has been desired. Therefore, a sialon phosphor has been proposed as a phosphor exhibiting little decrease of luminance instead of conventional silicate phosphors, phosphate phosphors, aluminate phosphors, sulfide phosphors, and the like.
The sialon phosphor is produced by the production process outlined below. First, silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3), and europium oxide (Eu2O3) are mixed in a predetermined molar ratio and the mixture is held at 1700° C. for 1 hour in nitrogen at 1 atm (0.1 MPa) and is baked by a hot pressing process to produce the phosphor (e.g., cf. Patent Literature 1). The α-sialon activated with Eu obtained by the process is reported to be a phosphor which is excited by a blue light of 450 to 500 nm to emit a yellow light of 550 to 600 nm. However, in the applications of a white LED and a plasma display using an ultraviolet LED as an excitation source, phosphors emitting lights exhibiting not only yellow color but also orange color and red color have been desired. Moreover, in a white LED using a blue LED as an excitation source, phosphors emitting lights exhibiting orange color and red color have been desired in order to improve color-rendering properties.
As a phosphor emitting a light of red color, an inorganic substance (Ba2-xEuxSi5N8:x=0.14 to 1.16) obtained by activating a Ba2Si5N8 crystal phase with Eu has been reported in an academic literature (cf. Non-Patent Literature 1) prior to the present application. Furthermore, in Chapter 2 of a publication “On new rare-earth doped M-Si—Al—O—N materials” (cf. Non-Patent Literature 2), a phosphor using a ternary nitride of an alkali metal and silicon having various compositions, MxSiyNz (M=Ca, Sr, Ba, Zn; x, y, and z represent various values) as a host has been reported. Similarly, MxSiyNz:Eu (M=Ca, Sr, Ba, Zn; z=⅔x+ 4/3y) has been reported in U.S. Pat. No. 6,682,663 (Patent Literature 2).
As other sialon, nitride or oxynitride phosphors, there are known in JP-A-2003-206481 (Patent Literature 3) phosphors using MSi3N5, M2Si4N7, M4Si6N11, M9Si11N23, M16Si15O6N32, M13S18Al12O18N36, MSi5Al2ON9, and M3Si5AlON10 (wherein M represents Ba, Ca, Sr or a rare earth element) as host crystals, which are activated with Eu or Ce. Among them, phosphors emitting a light of red color have been also reported. Moreover, LED lighting units using these phosphors are known. Furthermore, JP-A-2002-322474 (Patent Literature 4) has reported a phosphor wherein an Sr2Si5N8 or SrSi7N10 crystal phase is activated with Ce.
In JP-A-2003-321675 (Patent Literature 5), there is a description of LxMyN(2/3x+4/3y):Z (L is a divalent element such as Ca, Sr, or Ba, M is a tetravalent element such as Si or Ge, and Z is an activator such as Eu) phosphor and it describes that addition of a minute amount of Al exhibits an effect of suppressing afterglow. Moreover, a slightly reddish warm color white emitting apparatus is known, wherein the phosphor and a blue LED are combined. Furthermore, JP-A-2003-277746 (Patent Literature 6) reports phosphors constituted by various combinations of L Element, M Element, and Z Element as LxMyN(2/3x+4/3y):Z phosphors. JP-A-2004-10786 (Patent Literature 7) describes a wide range of combinations regarding an L-M-N:Eu,Z system but there is not shown an effect of improving emission properties in the cases that a specific composition or crystal phase is used as a host.
The representative phosphors in Patent Literatures 2 to 7 mentioned above contain nitrides of a divalent element and a tetravalent element as host crystals and phosphors using various different crystal phases as host crystals have been reported. The phosphors emitting a light of red color are also known but the emitting luminance of red color is not sufficient by excitation with a blue visible light. Moreover, they are chemically unstable in some compositions and thus their durability is problematic.
[Non-Patent Literature 1]
    H. A. Hoppe, and other four persons, “Journal of Physics and Chemistry of Solids” 2000, vol. 61, pages 2001-2006[Non-Patent Literature 2]    “On new rare-earth doped M-Si—Al—O—N materials” written by J. W. H. van Krevel, TU Eindhoven 2000, ISBN, 90-386-2711-4[Patent Literature 1]    JP-A-2002-363554[Patent Literature 2]    U.S. Pat. No. 6,682,663[Patent Literature 3]    JP-A-2003-206481[Patent Literature 4]    JP-A-2002-322474[Patent Literature 5]    JP-A-2003-321675[Patent Literature 6]    JP-A-2003-277746[Patent Literature 7]    JP-A-2004-10786
As conventional art of lighting apparatus, a white light-emitting diode wherein a blue light-emitting diode element and a blue light-absorbing yellow light-emitting phosphor are combined is known and has been practically used in various lighting applications. Representative examples thereof include “a light-emitting diode” of Japanese Patent No. 2900928 (Patent Literature 8), “a light-emitting diode” of Japanese Patent No. 2927279 (Patent Literature 9), “a wavelength-converting molding material and a process for producing the same, and a light-emitting element” of Japanese Patent No. 3364229 (Patent Literature 10), and the like. Phosphors most frequently used in these light-emitting diode are yttrium.aluminum.garnet-based phosphors activated with cerium represented by the general formula:(Y,Gd)3(Al,Ga)5O12:Ce3+.
However, there is a problem that the white light-emitting diode comprising a blue light-emitting diode element and the yttrium.aluminum.garnet-based phosphor has a characteristic of emitting a bluish-white light because of an insufficient red component and hence deflection is found in color-rendering properties.
Based on such a background, there has been investigated a white light-emitting diode wherein a red component which is short in the yttrium.aluminum.garnet-based phosphor is supplemented with another red phosphor by mixing and dispersing two kinds of phosphors. As such light-emitting diodes, “a white light-emitting diode” of JP-A-10-163535 (Patent Literature 11), “a nitride phosphor and a process for producing the same” of JP-A-2003-321675 (Patent Literature 5), and the like can be exemplified. However, a problem to be improved regarding color-rendering properties still remains also in these inventions, and hence it is desired to develop a light-emitting diode where the problem is solved. The red phosphor described in JP-A-10-163535 (Patent Literature 11) contains cadmium and thus there is a problem of environmental pollution. Although red light emitting phosphors including Ca1.97Si5N8:Eu0.03 described in JP-A-2003-321675 (Patent Literature 5) as a representative example do not contain cadmium but further improvement of their emission intensities has been desired since luminance of the phosphor is low.
[Patent Literature 8]
    Japanese Patent No. 2900928[Patent Literature 9]    Japanese Patent No. 2927279[Patent Literature 10]    Japanese Patent No. 3364229[Patent Literature 11]    JP-A-10-163535